The present invention relates to universal joints of the Cardan type, and more particularly to universal joints including a spider for use in the drive system of rolling mills.
As shown in FIGS. 3 and 4, universal joints of the Cardan type generally comprise a pair of yokes 1 splined or otherwise connected to a drive shaft and a driven shaft (not shown) respectively and rotatable therewith, a spider 2 provided between the yokes 1, and needle bearings 4 housed in the bores formed in the yokes 1 perpendicular to their axes to rotatably support the trunnions 3 of the spider 2. However, since the universal joints of this type have been provided with the development of motor vehicles, they are adapted for high-speed, low-torque rotation and are not suited for use in the drive system of rolling mills or the like which involves low-speed, high-torque rotation.
It is known that when a drive system incorporating a Cardan-type universal joint including needle bearings operates for torque transmission, the trunnions of the spider are subjected to fatigue bending stress and shearing stress and are therefore most susceptible to failure.
For a better understanding, the fatigue bending stress alone will be discussed with reference to FIG. 5.
Generally the fatigue bending stress .sigma..sub.B is expressed by: ##EQU1## WHERE
D: DIAMETER OF TRUNNION OF SPIDER.
D.sub.1 : DIAMETER OF OIL PORT IN SPIDER.
F: load.
H: LOADING LENGTH.
The torque to be transmitted, T, is expressed by: EQU T=F.times.2r (2)
where r is radius of force.
Equations (1) and (2) give ##EQU2##
The diameter d.sub.1 of the oil port in the spider, when raised to the fourth power, is so small as to be negligible as compared with the trunnion diameter d. Therefore it follows from Equation (3) that the fatigue bending stress is given approximately by ##EQU3##
Assuming that the fatigue bending stress .sigma..sub.B, the radius of force, r, and loading length h are constant, Equation (4) will be EQU T=c.multidot.d.sup.3
where c is a constant. Thus the torque T to be transmitted is in proportion to the third power of the trunnion diameter d. In other words, the torque T can be increased with increasing diameter d of the spider trunnion, namely with increasing shaft diameter, in proportion to the third power of the diameter d.
However, the conventional spider universal joints, which have been improved with the development of motor vehicles as already stated, are adapted to a high-speed, low-torque operation. They are designed for an increased angle of intersection, .alpha., namely they have an increased clearance S shown in FIG. 4. Because of such construction, the increase in the trunnion diameter d is limited. Moreover, with the joints of the type illustrated in FIG. 4 which include needle bearings 4 comprising needle rollers 4a and a needle case 4b, in other words with those fabricated from an increased number of parts, the increase of trunnion diameter d is more greatly limited.
Accordingly when such Cardan-type universal joint is used in the drive system of rolling mills or the like which involve low-speed, high-torque rotation, there arises the problem of insufficient amount of transmission of torque, consequently leading to insufficiency of the strength of the spider. In fact the convention universal joints of this type were not usable for a majority of rolling mills with a few exceptions where low torques are involved.
In order to clarify this more specifically, the relation between the torque T and allowable swing diameter D, i.e. T/D.sup.3, is calculated, and the value will be hereinafter referred to as a "torque factor T.sub.o." Listed below are the torque factor T.sub.o and angle .alpha. of intersection of Cardan-type universal joints of the prior art.
______________________________________ Torque factor T.sub.o Angle of (ton.multidot.m/m.sup.3) intersection(.alpha.) ______________________________________ For motor vehicles 80-120 25.degree. For agricultural machines 80-120 45.degree. For construction equipment 100-150 15.degree. ______________________________________
For use in the drive systems of various rolling mills, however, universal joints of the Cardan type require the torque factors T.sub.o given below.
______________________________________ Torque factor T.sub.o (ton.multidot.m/m.sup.3) ______________________________________ Hot strip mills 800-1400 Cogging mills and plate mills 400-600 Rod mills 150-250 Bar mills 150-300 Cold strip mills 200-400 ______________________________________
Thus the conventional universal joints of the Cardan type are barely usable for rod mills and bar mills, but they are unserviceable for other rolling mills and therefore applicable only to limited use.